Conventional white light emitting devices, such as those disclosed in United States Patent Application 2011/0186874 published Aug. 4, 2011 in the name of Shum and illustrated in FIG. 1, include a blue or UV emitting LED 1 mounted on or surrounded by an electric circuit 2, which provides electrical connections to an outside power source to power the LED 1. A metallic reflective layer 3, e.g. silver, is disposed on a silicon substrate 4 for reflecting any stray light refracted or reflected by the LED package back in the desired direction. Accordingly, an isolation layer 6 is also required to provide electrical insulation between the electric circuit 2 and the metallic reflector layer 3. Nitrides or oxides, such as Si3N4 or SiO2, are often used for the isolation layer 6 resulting in thicknesses of approximately 2 μm to 6 μm, depending on the legal requirements for the breakthrough voltages. Typically, the LED 1 is immersed in a light transmitting epoxy 7, which includes light converting dyes, e.g. phosphor, for converting the light emitted from the LED into a broad spectrum white light. Unfortunately, the silver reflective layer 3 is not environmentally stable over time and becomes tarnished, especially when exposed to high temperature and humidity, which greatly reduces the effective lifetime of the light emitting device.
The use of narrowband dielectric reflectors to reflect the particular wavelength of light emitted by the LED back through the phosphor material to maximize wavelength conversion has been disclosed in U.S. Pat. No. 6,833,565 issued Dec. 21, 2004 to Su et al; however, the problems of broadband, white light absorption/reflection and electrical isolation are not addressed.
An object of the present invention is to overcome the shortcomings of the prior art by providing a white-light emitting LED device in which a broadband, omnidirectional, multi-layer, dielectric reflector with varying thickness is used for providing reflectivity, thermal conductivity, and electrical insulation.